The present invention relates to a toner for developing electrostatic latent images including a thermoplastic resin and a black coloring material as main components and suited for use in a copying machine, in particular a copying machine comprising an oilless type, heated roller fixing device.
The oilless type heated roller fixing device herein referred to means the type of fixing device that comprises a pair of rollers, i.e. a silicone rubber roller and a heated roller of teflon-coated stainless steel, and has toner images present on copy paper thermally fixed thereon as the latter is passed between the pair of rollers.
As the black coloring material in the toner, it has been conventional practice to use magnetic powder such as of magnetic iron oxide or non-magnetic carbon black.
However, for obtaining a sufficient coloring only by means of magnetic powder, a magnetic powder content of 10 percent by weight or more is necessary. In ordinary magnetic brush development, the toner containing the above amount of magnetic powder, because of its own magnetism, undergoes a strong magnetic constraining force of a magnet roll, which tends to deteriorate transfer of the toner onto a photosensitive material (i.e. development) and lower a developed image density.
On the other hand, use of carbon black has advantages that there occurs no reduction of image density in magnetic brush development since carbon black is non-magnetic, that sufficient coloring is obtained with a relatively small amount, and that the tribo-electric charge of the toner is adjustable by varying the carbon black content. Therefore, carbon black has heretofore been in extensive use as the black coloring material.
However, the use of carbon black has the following disadvantage which is apparent particularly when toner images are fixed by the oil-less, type heated roller fixing device, and improvement has been desired.
The disadvantage is that, when images are fixed by the oil-less type heated roller fixing device, toner particles transferred onto copy paper in advance become unevenly distributed as they cool off after thermal fusion, which is particularly noticeable at so-called halftone image portions having about a 0.8 density, and which results in roughness in the texture of fixed images.